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Would anyone let me know whether the following formula is correct to estimate the total amount of DC link bulk capacitors for a 3-phase permanent magnet motor drive to handle the startup inrush current and regenerative breaking current?

  • C_bulk = (I_max * T)/dt

where,

  • C_bulk = Total amount of DC link bulk capacitance in uF
  • I_max = Assumed maximum reverse current during regenerative breaking (200A)
  • T = Switching period of the FETs (1/fsw) (120KhZ)
  • dt = maximum allowable ripple voltage (3V)

C_bulk = 555.28 uF (with the given data)

Is it correct? Should I add more around like 700 uF to Dc link?

I went through several research papers and they seem to use motor breaking current for the calculation.

I would much appreciate it if you would give me a direction or point me to any research paper considering the startup inrush current, normal phase current, and regenerative breaking current.

Nh K
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  • More capacitance = more inrush current (unrelated to the motor). Think about what I'm saying because this may end up being problematic. Can we have some numbers please? – Andy aka Mar 26 '23 at 15:11
  • say maximum phase current is 200 A, switching frequency varies from 40Khz or 100 Khz. Voltage ripple is 3V from the battery. – Nh K Mar 26 '23 at 15:54
  • I am using fets to limit inrush current.

    My inrush current is in KA range for 2 micro-sec in simulation which is down to 0A when I use the T_tao = RC principle for the capacitance charging circuit (Charge the DC link capacitance at 0.95*T_tao to limit the inrush current and charge with the battery voltage). I am using Rds_on (3mOhm) of FET as R here. But I can not figure out the FET turn-on time from Tao as less than 30ms cause a frequent charging cycle of the capacitors and cause a current spike of inrush current when the capacitor charges.

     – Nh K Mar 26 '23 at 16:07
  • I also need help to understand the relationship between the inrush current and the amount of capacitance. – Nh K Mar 26 '23 at 16:08
  • Add all relevant information into your question please. – Andy aka Mar 26 '23 at 16:10
  • Now, I added the numbers in the original question.

    Please also explain the relationship between the inrush current and the amount of capacitance.

     – Nh K Mar 26 '23 at 16:20
  • Capacitor inrush current – Andy aka Mar 26 '23 at 16:27

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